The invention relates to an electrical fuse for rotary current generators, having a rectifier assembly for supplying a DC network, in particular in motor vehicles, as generically defined by the preamble to the main claim.
In motor vehicles, to an increasing extent electrical devices are being used to improve the comfort and safety of the motor vehicles. For supplying these devices from the on-board electrical system of the motor vehicles, in by far the most cases, rotary current generators are used, which have a high power density and are thus under severe thermal loads. A rectifier assembly is as a rule integrated with the housing of the rotary current generators, so that only the DC output of this assembly forms the connection terminals of the generator, to which the connection cables of an accumulator battery of the on-board vehicle electrical system are connected. In the event of an overload or a short circuit at the generator or the rectifier assembly, which not infrequently happens in motor vehicles whenever a battery charging device is connected to the discharging battery with the wrong polarity, not only can the generator or the rectifier assembly be destroyed, but this can also trigger still further damage to the vehicle.
To avoid these dangers, it is already known from German Patent DE 30 01 522 C2, to dispose fuse elements in the rectifier assembly, between the connections of the positive and negative diodes of the three rectifier bridges. The fuses, which because of production variations have only very imprecise response values, are formed by line portions bent in a loop from recesses in a printed circuit board of the rectifier assembly; these loops melt open in the event of an electrical overload and can be temporarily repaired again by twisting the ends together. Since the terminals of the thus-protected power diodes of the rectifier assembly are soldered or welded to the printed circuit board, once a fuse has responded, the entire rectifier assembly later has to be replaced, which is expensive and time-consuming. Moreover, there is the risk that the fuse that has responded will be only temporarily repaired or in other words bypassed, without the rectifier assembly being replaced at the next opportunity. In that case, the risk of destruction and overheating increases quite considerably. If the response of a fuse in one of the three rectifier bridges goes unrecognized, the two intact rectifier bridges are then loaded all the more heavily, so that finally they fail as well.
The object of the present invention is to accomplish protection of the generator and the rectifier assembly from electrical overload or short circuit as simply and reliably as possible.
The electrical fuse of the invention having the characteristic of the body of claim 1 has the advantage over the prior art that when the fuse responds, the generator along with the rectifier assembly is shut off completely, making the response of the fuse clearly apparent from the interruption in the charging process. A further advantage is that if a battery charging device is connected with the wrong polarity, it is no longer necessaryxe2x80x94as it was until nowxe2x80x94for all three fuses in the three rectifier bridges to respond; instead, from the response of one fuse, the generator is completely disconnected from the on-board electrical system, thus averting the risk of overheating.
Advantageous refinements and embodiments are attained from the other characteristics recited in the dependent claims. For instance, an especially expedient embodiment that is simple from a production standpoint is obtained if the fuse element is disposed between a heat sink, forming the positive pole, of the bridge rectifier and a positive connection bolt of the generator. The positive connection bolt is fastened in insulated and detachable fashion in a hole of the positive heat sink.
For the sake of changing the fuse as easily as possible if the fuse has responded, it is expedient to slip the fuse element, together with an insulator part embracing the head of the positive connection bolt, onto the positive connection bolt and to be put into electrical contact with it on the face end of the head. For the sake of the most reliable possible contacting, it is also proposed that the positive connection bolt, insulator part and fuse element form a prefabricated component unit, in that the fuse element is galvanically connected, preferably welded, by one terminal, to the connection bolt head, on the face end thereof. In this case, the complete component unit is replaced after the fuse has responded.
The most reliable and simple connection of the fuse element to the positive heat sink is attained in that with its other terminal, the fuse element is embodied annularly and is placed on a collar, embracing the connection bolt, of the insulator part, and this terminal, on its top side, is put into electrical contact with the positive heat sink by pressure when the connection bolt is tightened.
Very simple production of the fuse element can be achieved in that the fuse element is a stamped metal part, which between its two terminals has a portion of narrowed cross section, embraced laterally on the head of the positive connection bolt by the insulator part, which portion melts away in the event of an overload.
To prevent a direct connection of the positive heat sink to the positive connection bolt and thus to prevent bypassing of the fuse element, an insulating disk is advantageously placed on the positive connection bolt above the positive heat sink, in order to insulate the heat sink from a protective cap secured in a manner known per se to the connection bolt.
As alternative to the aforementioned replacement of the entire component unit with the fuse element, the embodiment can expediently also be designed in terms of its construction such that only the fuse element has to be replaced after its response. In that event, it is provided that the positive connection bolt is embraced at its head region by an insulator part and protrudes in insulated fashion through the hole in the positive heat sink, the fuse element being placed on the positive connection bolt in such a way that with its lower terminal, it rests on the positive heat sink, and with its upper terminal, it is put into electrical contact with the positive connection bolt by means of a nut.
Also in this respect, it is provided that the fuse element is embodied of an annular, disklike insulating substrate body, which on each of its two face ends has a respective annular-disklike terminal, and a melting portion of narrowed cross section is moved laterally past the insulating substrate body between the two terminals.
For the sake of safety, to make it possible to fasten the portion that melts away upon response of the fuse in a predetermined position, secure against rotation and positionally securely, on the positive connection bolt, this portion is guided around a platelike protrusion, formed laterally onto the insulating substrate body, that embraces the melting portion on both sides. In addition, this protrusion is positionally fixed on both sides by lugs that are formed onto the positive heat sink.
Since this fuse element can also be put on the market as a spare part, the insulating substrate body must not fall out between the two terminals of the fuse element before installation. To that end, it is provided that the two annular-disklike terminals of the fuse element embrace the insulating substrate body on its outer edge, in each case by means of angled tongues.